"Beloved, while I was very diligent to write to you concerning our common salvation, I found it necessary to write to you exhorting you to contend earnestly for the faith which was once for all delivered to the saints" (Jude 3).
The words "for all" is not in the King James version of the Bible, which I use.
When translating from one language to another it is often difficult to get the subtle nuances of words to carry across. It is for this reason that sticking with just one translation will not give you the depth of understanding that you could gain by examining multiple translations or by going back to the original language.
"Once" is used in the King James Version. "Once for all" appears in most modern literal translations, including the New King James Version, the New American Standard Version, the New International Version, the World English Bible, and the English Standard Version. The Greek word being translated is hapax, which The Complete Biblical Library defines as:
"From the time of Homer, classical Greek writings contain this term as an actual numeral. However, this word, like the Latin semel, also could refer to the quality of perpetual validity, that which did not need repetition. Sometimes it was employed in connection with the enactment of a law that was given once and for all. This once-for-all sense came to be far more important than the simple numerical significance ... The major impact of this expression can be seen in the New Testament. While it sometimes is seen as "once" in contrast to two or three times (2 Corinthians 11:24; Philippians 4:16; Hebrews 9:7), its major significance relates to the uniqueness of the sacrifice of Christ as something that cannon and need not be repeated. both the writer of Hebrews (Hebrews 9:26-28) and Peter (1 Peter 3:18) used this adverb to express the once-for-all nature of God's actions, especially as related to the sacrifice of Jesus for mankind. While the high priest had to enter the Most Holy Place every year with a blood offering, Jesus did so only once. Furthermore, the high priest first had to sacrifice for his own sins, but Jesus had none. Jude also employed this term to refer to the once-for-all body of doctrine given to the Church (Jude 3)."
In this case the King James Version is not as precise as most of the new translations.